The present invention relates to the generation of inductively coupled plasmas in apparatus for performing etching and deposition processes.
A variety of semiconductor fabrication operations involve deposition and etching processes performed on a semiconductor substrate mounted within a process chamber. Such processes typically involve the use of a low pressure, high density discharge wherein a plasma is generated by the interaction of an ionizable gas with a radio frequency (RF) electromagnetic field. The coupling of RF power to a plasma in semiconductor process chambers can be categorized as either predominantly capacitive or predominantly inductive. Many examples of each can be found in the prior art.
In the case of capacitive coupling, RF power is coupled to the bottom plate and/or the top plate of a parallel plate process chamber. In general, the top plate also serves as the ionizable gas feed, the bottom plate serves as the wafer holding chuck and the remainder of the chamber is grounded.
Inductive coupling generally employs a planar geometry, or a cylindrical geometry, or a combination of the two geometries. Furthermore, low RF power is usually applied to a bottom electrode, or chuck, to provide a RF bias. FIGS. 1A, 1B and 1C present some examples of the inductive discharge geometries.
FIG. 1A illustrates an example of planar geometry in which a planar multi-turn coil is located at the top of a process tube, or process chamber. FIG. 1B shows an example of cylindrical geometry in which a multi-turn cylindrical coil is wound around a process tube, while FIG. 1C shows a modified version of cylindrical geometry in which the cylindrical coil is surrounded by a conductive shield. The structure shown in FIG. 1C is an example of a helical resonator. In each of the illustrated arrangements, the coil is connected to receive a RF current and, thence, to induce an electromagnetic (EM) field parallel to the longitudinal axis of the cylindrical geometry. This resultant RF EM field, that is a manifestation of the RF current in the coil, consists primarily of radially propagating EM waves proximate to the plasma volume when polarized by an electrostatic shield (to remove the azimuthally propagating field). The radially propagating waves interact with a small thin surface layer of the bulk plasma. The thickness of this thin layer is often referred to as the skin depth. This interaction ultimately leads to energized electrons and subsequent gas ionization, and the formation of a plasma. In general, a process tube acts as a protective barrier and delineates the inner plasma volume from the external structure. At least in the structures of FIGS. 1B and 1C, the process tube is made of a dielectric material that is transparent to the electromagnetic energy emanating from the coil. It will be understood that these figures are schematic. Actual equipment can take a variety of forms in practice.
The coupling of RF power to a plasma in semiconductor processing is conventionally at a drive frequency of 13.56 MHz, using a 50 xcexa9 RF power generator. This frequency is conveniently located within a RF band designated for industrial use. However, the frequency of operation is not limited to this value in the prior art and, in fact, multiple frequencies are employed typically when using multiple coupling electrodes.
RF power is typically supplied to the coil by an oscillator having at least one active component that may be a solid state, or semiconductor, component, or a vacuum tube.
As is known in the art, energy can be inductively coupled into a process chamber through a helical resonator as described in Lieberman and Lichtenberg, Chapter 12 (Principles of plasma discharges and materials processing, John Wiley and Sons, Inc., 1994). With a helical resonator, the coil (or helix) has a length equal to an integral number of quarter waves of the RF input. The coil surrounds the plasma chamber and is encased within a cylindrical container that is grounded. FIG. 1C shows the basic structure of such a helical resonator including the coil, an electrostatic shield enclosed by the coil to minimize capacitive coupling of the RF field with the plasma, a dielectric process tube that is enclosed by the electrostatic shield and separates the helical coil from the plasma, an outer conductor, or shield, surrounding the coil and an RF input line connected to a tap of the coil. As shown in FIG. 1C, the coil tap to which the RF input is applied is spaced from one end of the coil which is grounded. The portion of the coil between the coil tap and ground effectively serves as part of the matching circuit, thus the tap position can be selected to achieve a match condition. Under a given set of conditions, proper definition of the tap point location can provide impedance matching for the circuit.
However, the load impedance on a RF power generator is a function of the intrinsic impedance of the coil and the impedance presented by the plasma, the latter impedance being a function of the properties of the plasma. Therefore, fluctuations in the process conditions can lead to fluctuations in the impedance as seen by the RF power generator. Furthermore, the impedance of the process chamber, in which the plasma is established, varies significantly between the condition prior to plasma ignition and the run condition. In order to maintain efficient energy transfer from the RF power generator to the plasma, proper matching of the power supply output impedance to the load impedance is required.
One technique used in the prior art is a variable frequency power supply. The frequency is determined by a phase mag detector that determines the match conditions at the input of a fixed match network coupling to the tap of the coil. However, systems of this type can be very expensive, and hence a fixed frequency power supply is generally employed in conjunction with a match network.
An example of a fixed frequency RF oscillator coupled to the coil of a helical resonator via an impedance matching network is shown in FIG. 2. The matching network is a xcfx80-filter composed of a series connected inductor, L, and two shunt connected variable capacitors C1 and C2. The matching network compensates for differences between the variable load impedance represented by the coil and the plasma, and the output impedance of the RF power generator. For example, as shown in FIG. 2, when the source impedance Zs is equal to the load impedance ZMNi, this impedance including the impedances of the match network, the helical coil and the plasma load, then the power transfer can be maximized. In this particular case, the input impedance to the match network-load circuit ZMNi is the complex conjugate of the source impedance Zs, and the output impedance of the match network ZMNo, as seen by the load, is the complex conjugate of the load impedance ZL. Under this special condition, the coupling between the RF source and the combination of the match network and the plasma loaded coil can be represented as equivalent to a purely resistive circuit. Hence, the matching network is designed to maximize power transmission from the RF power generator to its load.
Given feedback of the power transfer state (reflected/transmitted power levels using special detector circuits whose outputs approximate the difference in phase between the forward and reflected signals and the magnitude of the reflected signal), matching networks have been developed to respond to changes in the load impedance. In particular, during plasma ignition and run conditions, the variable capacitors are adjusted to tune the load circuit, which includes the impedance match network, the coil and the plasma load, to a resonant condition for the fixed frequency power supply. When the circuit impedances are matched, power reflected to the source at the match network juncture is minimized, or even zero, depending upon the accuracy of the match, thus reducing damage to the power supply, which must ultimately absorb this reflected power. It is known, however, that the use of a matching network with a fixed frequency power supply presents a number of problems for the manufacturers of semiconductor equipment.
Specifically, existing impedance matching networks are inherently unreliable, due in part to the fact that the maintenance required to assure operating reliability is relatively complex and often beyond the capabilities of maintenance personnel.
Furthermore, know n matching networks have an inadequate response time, at least in certain operating situations. In particular, if the power supplied to the plasma source is to be varied according to a pulse pattern, then the fastest matching networks cannot adjust to maintain an optimum match between the power supply and the plasma source. This is true because the time scale for the fastest match networks is several hundreds of milliseconds, i.e., the rise or fall time for a response is approximately several hundred milliseconds. However, to achieve a RF square wave pulse to within one percent accuracy, the minimum pulse time scale for these match networks might be several tens of seconds or 25 to 50 seconds. Therefore, in order to accurately achieve millisecond pulsing, one requires a match network with rise and fall times, or a time scale, of the order of microseconds. Therefore, it has been necessary to accept power coupling conditions that are inefficient and that are even variable from pulse to pulse or from run to run.
If an impedance mismatch should occur during substrate processing, substrate damage will be the likely result.
The use of a variable frequency RF power generator alleviates many of the problems encountered when employing a fixed frequency RF power generator and a match network. U.S. Pat. No. 5,688,357 (Hanawa) discloses a method of using a variable frequency RF power generator composed of a solid state oscillator in conjunction with a control system that includes a method of sensing the reflected and/or transmitted power. The control system adjusts the frequency of the RF power source until the reflected power is minimized and/or the transmitted power is maximized. A disadvantage of present solid state technology is the fact that RF power supplies having a solid state component are suitable for handling relatively low power levels, of the order of 5 kW. However, power supplies capable of generating higher power levels, for example up to 15 kW, are necessary to process wafers having diameters of 300 mm. An alternative to the use of a solid state oscillator is the use of a vacuum tube as the active component within an oscillator circuit that includes the load coil and plasma load.
Vacuum tube oscillators have been employed for more than 50 years to convert direct-current (DC) power to alternating-current (AC) power. A complete discussion of the design of vacuum tube oscillators may be found in xe2x80x9cVacuum-Tube Oscillatorsxe2x80x9d (Chapter XI of Principles of Electrical Engineering Series Applied Electronics, A First Course in Electronics, Electron Tubes and Associated Circuits by Members of the Staff of the Department of Electrical Engineering, MIT, John Wiley and Sons, Inc., New York, 1943). According to the literature, vacuum tube oscillators have been categorized in two classes, namely negative-resistance oscillators and feedback oscillators. For particular use in the processing of semiconductors using low pressure plasma discharges, feedback oscillators can comprise a vacuum tube as an amplifier and a coupling circuit wherein the coupling circuit includes the load coil, which may be a helical coil or electrical components that couple the RF power with a plasma. Examples of typical feedback oscillators are the: Hartley oscillator, Colpitts oscillator, tuned-grid oscillator, and tuned-grid tuned-plate oscillator. Basic circuits for known Hartley and Colpitts oscillators are shown in FIGS. 3A and 3B, respectively, which are found in Vacuum-Tube Oscillators, supra.
The basic premise behind the operation of a feedback-oscillator is that the device acts as an amplifier wherein a portion of the output power is fed back as input to the amplifier such that oscillations may be maintained. Hence, any device capable of a periodic output with an output power greater than the input power required to drive the oscillations may be referred to as self-excited. More precisely, if a component of the output power is fed back to, for example, the cathode of the vacuum tube with the proper magnitude and phase, then oscillations can be sustained. Sometimes, it is useful to view the feedback connected vacuum tube oscillator as a negative-resistance element.
FIGS. 4A and 4B present a simplified schematic diagram and an equivalent circuit diagram, respectively, of a feedback oscillator corresponding to the Hartley oscillator shown in FIG. 3A. FIGS. 4A and 4B are also found in Vacuum-Tube Oscillators, supra. In FIG. 4A, the circuit is composed of a vacuum tube amplifier and a coupling network. As shown, the vacuum tube amplifier has an output voltage Ep (plate to cathode), an input voltage Eg (grid to cathode), and a voltage gain K=Ep/Eg. The coupling network sees an input voltage Ep and has an output voltage of Efb where xcex2=Efb/Ep is the voltage ratio of the coupling network. In order to generate self-excited oscillations, the voltage gain of the amplifier K must be at least equal to the inverse of the feedback voltage ratio xcex2, or Kxe2x89xa71/xcex2.
FIG. 4B presents an equivalent circuit diagram of the same circuit as FIG. 4A. However, it assumes the circuit to be a linear Class A circuit. Substituting for the value of K in the circuit of FIG. 4B, it is possible to show the following condition for sustained oscillations, commonly referred to as the Barkhausen criterion,                                           β            ⁡                          (              ω              )                                =                      (                                          1                μ                            +                              1                                                      g                    m                                    ⁢                                      Z                    ⁡                                          (                      ω                      )                                                                                            )                          ,                            (        1        )            
where Z is the impedance of the load circuit, and xcexc and gm the gain and the mutual conductance, respectively, of the vacuum tube. Clearly, xcex2 is a complex voltage ratio, since the impedance more than likely includes reactive components, wherein the real and imaginary parts must be independently equal to satisfy equation (1). These two criteria place constraints on the magnitude and phase, and hence define a necessary condition for operation. In fact, sometimes the real part of equation (1) sets the condition for the mutual conductance of the tube gm, and the complex part of equation (1) generally sets the frequency of operation.
As shown in FIG. 3A, the load circuit of the Hartley oscillator whose impedance is Z comprises two inductors L1 and L2 in parallel with a capacitor C, wherein the common node between the two inductors is directly connected to the vacuum tube cathode.
In connection with plasma generation in spectrometers, European patent EP 568920A1 (Gagne) discloses the use of a triode vacuum tube within a Colpitts oscillator circuit for coupling RF power to an atmospheric plasma. However, the oscillation circuit is disclosed as having a poor efficiency, approximately 40 to 60%, for coupling power to the plasma. Additionally, when designed as a feedback oscillator for coupling RF power to a low pressure, high density plasma, the Colpitts oscillator was unable to transition from a plasma ignition condition to a run condition without manual circuit tuning. In order to overcome these problems and improve the robustness of the oscillator circuit, a Hartley oscillator has been employed to enable automatic transition between start and run conditions. Furthermore, the Hartley oscillator circuit was found to be more efficient; approximately 78%.
It is an object of the present invention to provide a plasma generating system with a RF power generator which alleviates the drawbacks and shortcomings noted above.
Another object of the invention is to provide such a system with a RF power generator that can generate high power levels, in the range of 15 kW and higher, but is less costly than existing power supplies capable of operating at such power levels
A further object of the invention is to provide a RF high power generator that is capable of power transfer to the plasma source, while adjusting rapidly to changes in the RF power level, e.g. with match network time scales of the order of one or several microseconds, and continuously maintaining a matched impedance coupling circuit during variations in the plasma source impedance.
It is a further object of the invention to provide a RF generator that operates stably during start and run conditions and is capable of automatic transition between start and run conditions without manual tuning of the circuit.
The above and other objects are achieved, according to the present invention by a system for converting DC power into a RF electromagnetic field in a processing chamber, the system comprising:
a coil constructed to surround the processing chamber for coupling RF power into the plasma; and
a RF power generator including a free-running oscillator having a DC power supply and an RF power output, the power output being connected to a load impedance which includes the coil, the RF power generator being operative for supplying RF current to the coil in order to generate the RF power that is coupled into the plasma, wherein:
the free-running oscillator comprises: a vacuum tube having a cathode, a plate and a grid; a grid-leak circuit connected to the grid; a feedback circuit coupled to the vacuum tube; and a DC supply circuit connected for heating the cathode; and
at least a part of the coil is connected to form a part of the feedback circuit.
According to the preferred embodiments of the invention, the RF power generator is located proximate to the processing chamber, while the DC power supply may be disposed at a location remote from both the processing chamber and the RF power generator. However, the DC power supply may be placed at any location, including adjacent to the chamber. The size of the DC power supply is a function of the required RF power level and, hence, it can become quite large when 15 kW RF power is to be generated for wafer processing.
In order to achieve the desired RF power output levels, i.e. RF power ramps or pulsed RF power, a RF power generator according to the invention may utilize, as its active component, a triode vacuum tube having a control grid wherein the grid-bias voltage can be modulated using a wave-form generator. The active component can also be a tetrode, pentode, etc having single or multiple control grids, solid state transistors, FETs, or similar gain devices. According to preferred embodiments of the invention, the vacuum tube free running oscillator is a modified Hartley oscillator.
It has been found that the use of a free-running oscillator eliminates the need for any matching network and allows the oscillator output to be directly connected to the inductive coil, and thus directly coupled to the plasma source.
When conditions change in the processing chamber, there will be a corresponding change in the plasma impedance, and hence a corresponding change in the load impedance of the oscillator circuit. In addition to being a function of the vacuum tube parameters, the frequency at which oscillations are sustained is a function of the load impedance. Therefore, during stable operation, the RF frequency of the free-running oscillator will automatically adjust to such changes.
A system according to the present invention offers a number of advantages over the prior art, including improved reliability, consistency of performance from one unit to another, speed of response to power level variations, and the ability to monitor plasma conditions by measuring the oscillator output frequency.
The ability of a free-running oscillator to vary its oscillation frequency rapidly with changes in load impedance, without relying on mechanically adjustable components, results in a more reliable matching of the RF power generator output impedance to the impedance of the plasma source and thus allows a more accurate control of the power delivered to the plasma. In addition, the elimination of mechanically adjustable components and related control circuits significantly improves system reliability and reduces system fabrication costs.
In other words, the response of the RF feedback oscillator is dependent only on the rate at which the plasma impedance can change and the time constants of various circuit elements. For example, the time scale associated with such changes in plasma impedance may be tens of microseconds during run conditions, and as long as 1 millisecond during start operation. Thus, the use of a free-running oscillator allows efficient transfer of power to the plasma source even when the RF power is supplied in the form of pulses having a duration of 3 to 5 milliseconds, or less if 1% accuracy is not required, and with rise and fall times as short as 30 and 50 microseconds, respectively. The RF power generator can respond satisfactorily to power level variations and pulses having a duration of 3 to 5 milliseconds and can operate satisfactorily even when the RF power is pulsed between different power levels with a complex, multi-level, cyclic or non-cyclic time function.
Furthermore, since the RF frequency produced by the RF power generator varies with the plasma source impedance, the RF frequency can be monitored to provide an indication of variations in plasma conditions. This can be particularly helpful to the extent that the RF frequency can be correlated to specific plasma source parameters.
Although the present invention offers numerous advantages, it is in order to note that, in the present state of the art, high power vacuum tube RF oscillators require regular maintenance performed by highly trained personnel. In addition, under certain operating conditions, the oscillator output frequency may be modulated at frequencies outside the ISM Band designated for industrial RF use. However, this problem can be alleviated by proper RF shielding around the power supply and process chamber. This RF shielding may, for example, be in the form of a copper mesh and/or a copper plate wall, enclosing the plasma chamber.
This shielding is not specifically shown in the drawings. However, in accordance with principles already known in this art, all the RF components, namely, the coil, cables, the RF oscillator and oscillator circuit elements would conventionally have appropriate shielding, i.e., would be enclosed within metal boxes, coax cable, etc. There may still be some RF leakage, and if its magnitude is sufficiently large then further shielding may be required, possibly in the form of a copper plate wall and/or mesh shielding the entire chamber. RF shielding is standard practice for most process chambers.